[louieatienza]

It's been a while since the last update, so here it goes. I have been testing all kinds of stuff to see what the machine (and myself) manage to make. My 'End Mills' from ebay for testing have been 4/6/8mm 2flute ones. And managed to burn the 6mm one 2 weeks ago. All end mills are HSS for price reasons, so I will need some good ones soon.

Now after my tests I discovered multiple problems and fixed most of them. I reinforced the Y-axis in multiple places and it seems to be better, but there is one thing I can't figure out at all. I don't even know where to looks, so I hope someone here has an idea (I'm still a complete newbie, so it could be obvious to you guys).

The attached pictures show my last 'test' in black MDF. I did cut the same thing twice in one job, next to each other. Pic 3 shows the one that has been completed 'almost OK', pic 1 and 4 show the 'not OK version'. It looks like the first pass is completely off on both pieces and it gets restored from the second pass on. The strange thing is that I haven't seen this on other materials like in pic 2.

Is there someone here who has an idea where to start looking ? Gcode is made with vCarve and the controller is GRBL, I don't think that is a problem. I guess there still is some design/build error, but if I was missing steps I think the problem would get worse (which isn't the case at all) ?

Second guess was to much flew on the Y axis, but in pic 4, the 'bump in the long side' is away from where it should be, so that sound strange also.

I'm also sure my HSS bit at 12000rpm (Kress milling engine) isn't the correct one, but to produce things that look that bad :/ ?

Hope someone can point me in some direction ... I made multiple thing that look perfect, but with these big jobs (far away from the starting point) it going really wrong

I would say get some solid carbide endmills, they're stiffer and stay sharper longer, and more resistant to heat. Speaking of which, if you're using a 2-flute (or more) bit you might want to consider a single flute bit and higher spindle RPMs. With the smaller bits the Kress accepts making the appropriate sized chips can become an issue. If you just make dust not chips they'll jam up between the bit and the material and the bit will rub more than cut, then heat up causing dulling whichmakes the problem worse. Not knowing your feedrate, you might have to lower your DOC and increase your feedrate significantly.

On the bit side, don't believe that you're saving money buying inexpensive HSS bits in the hopes that you'll learn from them, especially when you end up producing parts that you cannot use! They work great on mills that have way lower spindle RPMs and handheld use where you don't have as fast a feed as a CNC. Successful wood machining on the CNC requires higher feedrates blended with the optimum spindle speed.

As to clip clearing, this is important too, especially with lower powered spindles like a Kress (and as I suspect multi-flute bit) clearing chips with either dust collection, vacuum, or compressed air. Though a single flute bit will create larger chips that get "thrown" from the cut area easier, and a multi-flute bit not run fast enough will create dust that is harder to remove.

[P.Passuello]

Hi MrChronoM,

To be honest it looks like lost steps to me but I'm not sure. Fortunately there is an easy way to check. Before you start cutting make a mark with the cutter at X0,Y0 or if this will spoil your workpiece pick a coordinate that is in the waste stock area. Once the cut is finished jog back to 0,0 or other chosen location and check if the cutter is still on the mark. If not you have lost steps and can start looking for them. If the cutter lines up properly on a failed cut then at least you know what it isn't.
Stepper motors don't run properly on 24 volts especially a 280 oz motor. Depending on the motor specs 50 volts is more likely. I run 270 oz motors and can rapid at 600ipm The previous driver I had on mine used 24 volts, 50ipm was maximum and loosing steps was standard practice. Same motors, different driver, unbelievable difference.

Another possibility is flex in your carriages. I know you said you beefed up the Y carriages but any flex in your carriages will be punished. By the look of the error in photo 4 I don't think you could miss that amount of flex.

You also mention you burnt out a cutter. I looked at the last video you posted and the machine was moving very slow. With the cutter running at 12000 rpm I would expect the machine to be running at 100 inches per minute minimum. By running the cutter too fast and the machine slow the cutter ends up spinning in one position and rubbing on the material building up heat, burning the stock and cutter.

I hope this helps
Peter

[Mr.ChronoM]

This is really helping me understand multiple issues I have seen on the last (black MDF) piece, so thanks! It's also clear that it is way harder to use a machine like this, then it is to build one. I'm gone try and find some good tutorials before I fire it up again as I have never used any type of CNC machine before, so after only 6-7 small pieces, this is still a big learning curve.

Maybe some more information from my side on the above problem. A few pieces ago I 'decided' that it was a good idea to lower the feed rate from 2000mm/min to 1000mm/min and keeping the 15000rpm on the spindle (I started at 25000rpm for earlier pieces in plywood until I noticed that the bit was black after 2 minutes :/ ). Running that in 8mm plywood didn't burn it, so I figured that was OK.
Running the same setting in MDF, as I did yesterday made the 'chips' become dust and because of your explanation I now understand that this heated the cutter (making it dull as it is today). To make things worst my vacuum cleaner almost stopped working because of the dust instead of chips, holding the dust inside of the cutting area.
The HSS bits are indeed inexpensive, but 40-50€ for about 3-4 bit I 'need' was simply to much for a complete beginner. I know understand that I really will have to get something good, starting with an 8mm version, because that is what I use most. Any good webshops for metric sized bits for 6mm and 8mm collet ?


Now for 'solving' that lack of knowledge problem ... I have read multiple sites/blogs about spindle speeds that all say 'It will be different on your machine'. Can someone point me to a good piece to read that explains how I can get to a good setting ? I have been tempted to buy 'G-Wizard', but this machine is getting expensive. Of course trail and error has also proven not to work as well as I planned before starting this cnc thing !

My machine can run feed rates to around 2000-2500mm/minute (just running from a to b). The spindle speed can be set from 10000rpm up to 29000. I have some HSS end mills in 1-2-4-6-8mm that are all 2 flutes. The 6 and 8mm are burned as of yesterday . The goal is to cut MDF and plywood in 4-6-8-18mm at this point (Acrylic and HDPE is for a later stage).

I would say get some solid carbide endmills, they're stiffer and stay sharper longer, and more resistant to heat. Speaking of which, if you're using a 2-flute (or more) bit you might want to consider a single flute bit and higher spindle RPMs. With the smaller bits the Kress accepts making the appropriate sized chips can become an issue. If you just make dust not chips they'll jam up between the bit and the material and the bit will rub more than cut, then heat up causing dulling whichmakes the problem worse. Not knowing your feedrate, you might have to lower your DOC and increase your feedrate significantly.

On the bit side, don't believe that you're saving money buying inexpensive HSS bits in the hopes that you'll learn from them, especially when you end up producing parts that you cannot use! They work great on mills that have way lower spindle RPMs and handheld use where you don't have as fast a feed as a CNC. Successful wood machining on the CNC requires higher feedrates blended with the optimum spindle speed.

As to clip clearing, this is important too, especially with lower powered spindles like a Kress (and as I suspect multi-flute bit) clearing chips with either dust collection, vacuum, or compressed air. Though a single flute bit will create larger chips that get "thrown" from the cut area easier, and a multi-flute bit not run fast enough will create dust that is harder to remove.

[Mr.ChronoM]

If would start over I would think of getting another controller, but at this point I will have to do with the GRBLShield that can't really handle more then the 24v PSU.

The limiting factor in going faster is the machine itself, it's build with handtools and not perfect. Building a new version is on the list, but then I have to fix my MDF cutting issues first héhé . I do need to test the machine again at > 100 IPM since the changes I made. 2000mm/min (80 IPM will be the max at this point I guess now)

@All => Thanks for the replies !
My next tests will be at a higher feed rate and still at 12000rpm then. I need to get a new cutter anyways, so following the above advice and multiple other threads, I guess I need to get a 1 or 2 flute carbide cutter right ?

Something like this maybe ?
8mm x 20mm LOC SOLID CARBIDE SINGLE FLUTED CNC ROUTER END MILL CUTTER GBR #B13 | eBay
http://www.ebay.co.uk/itm/Solid-carb...#ht_783wt_1163
http://www.ebay.co.uk/itm/6mm-x-16mm...ht_2460wt_1163